422 GEOLOGY OP OHIO. 



front of the valve as the length from beak to base. Surface of the valve marked 

 on each side of the sinus, by two low, angular, but distinct plications, besides those 

 bordering the sinus ; no other markings are traceable on the surface of the shell. 

 The margin of the valve between the plications is extended, forming rounded pro- 

 jections similar to that of the mesial sinus, and probably corresponding to low 

 rounded plications which have characterized the dorsal valve, which has not been 

 observed. 



The broad sub- triangular form of the shell, with the shallow ventral 

 valve and the small number of low, angular plications, will readily dis- 

 tinguish this from any species hereto known. There may possibly be 

 some doubt as to the generic reference of the species; but this cannot be 

 positively determined until more perfect individuals are obtained. 



Formation and Locality. — In limestone of the Upper Helderberg 

 group, at Smith and Price's quarries, near Columbus, Ohio. Collected by 

 the Hyatt brothers, of the State University. 



MOULUSCA. 



LAMELLIBRANCHIATA. 



Genus MYTILARCA H. and W. 



Prelim. Notice Lamellibranchiate Shells, Up. Held., Ham. and Chemung Groups 

 etc. State Cab. Nat. Hist., Dec. 1869. 



Mytilarca percarinata. 



Pl,ATE VI, figs. 1 and 2. 

 Mytilarca percarinata Whitf., Ann. N. Y. Acad. Sci., March, 1882, p. 202. 



Shell less than medium size, the specimen used for description and illustration 

 measuring but one and three-fourths inches in extreme height; and the distance 

 from the anterior to the posterior margins across the point of greatest diameter, 

 only a trifle over one inch; the depth of the valve being nearly half an inch. Form 

 of the shell elongate triangular-ovate, rather acutely pointed at the beak, which is 

 small and incurved ; anterior, or byssal, margin straight and absolutely vertical in 

 the example mentioned; basal margin broadly rounded from the anterior line 

 nearly to the point of greatest length of the valve, where it is more rapidly curved, 

 and finally passes abruptly into the rapidly ascending posterior margin ; the lower 

 part of which is nearly parallel to the anterior side, but above inclines more rapidly 

 toward the short and very oblique hinge-line. The surface of the valve is most 

 elevated along the anterior umbonal ridge, where it is at right angles to the ante- 

 rior surface, but slopes gently backward for two-thirds of the distance toward the 

 posterior margin, and on the other third much more abruptly. Near the beak, the 

 surface rounds rapidly from the anterior ridge to the posterior border. Surface of 

 the shell marked by numerous concentric ridges, parallel to the margin of the valve, 

 many of which are strongly marked and form varices of growth. On the anterior 

 surface, these varices and the concentric striae are well marked. Cardinal area not 

 observed. 



The example used is a right valve, and bears evidence in its charac- 

 ters of being an adult shell. It is associated in the same layers of cherty 

 material with M. ponderosa, H. & W. (Prelim. Notice I^amell. Shells, etc., 

 p. 21), but may be readily distinguished by the vertical anterior surface 

 and the angular umbonal ridge. From the young of that species, it is 



